1. Field of the Invention
This invention relates to the use of latex compositions as coatings, including backcoatings, sealants, adhesives and the like, and particularly to coatings including ring-halogenated, ethylenically unsaturated aromatic monomers and at least one other monomer.
2. Description of the Prior Art
Acrylic, styrene-butadiene, styrene-acrylic, vinyl-acrylic, and vinyl acetate latices are commercially used in a variety of coatings. In many coatings applications, the latices used are desired to have flame-retarding properties. This applies in particular where latices are used in textiles, carpeting, paints, clear coatings, adhesives, sealants, caulking, non-woven binders and so on.
The usual method by which flame-retardant properties are imparted to latices is the blending-in of flame retardant additives. Many of these flame retardant additives contain bromine, such as brominated diphenyl or diphenyloxide compounds, together with antimony trioxide. However, such flame retardant additives have a major disadvantage in that their use gives rise to problems, such as the generation of strong white pigmenting and settling out effect, and toxicity resulting from the presence of antimony trioxide.
A common approach has been the addition of solid organic and inorganic compounds to latices to confer flame retardancy. U.S. Pat. No. 3,877,974 describes the admixture of an aqueous dispersion of a halogenated organic compound and metallic oxide with a polymeric adhesive binder. Although this approach has been shown to provide the desired flame retardancy, many undesirable features are again introduced. Solids ultimately separate from the latex emulsion despite any dispersion techniques employed. The dispersions tend to be high in viscosity and impede application of the latex. Latex coatings become stiffer due to the presence of solids, interfering with the flexibility or "hand" of the latex. In addition, solids tend to have a pigmenting effect which masks or changes the color of the substrate.
Liquid compounds have been added to latices as well. U.S. Pat. No. 3,766,189 teaches the use of liquid chlorinated paraffin in a latex to achieve fire retardancy. Drawbacks to the use of liquids include migration from the polymer with time, separation from the liquid latex emulsion, adverse effect upon adhesion, plasticizing, swelling of the latex, and poor water resistance. Salts and other water soluble solids eliminate the problems of settling of solids, but contribute other problems cited as well as generally having an adverse effect upon the stability of the latex emulsion.
Chemical integration of monomers into latex polymers to impact flame retardancy has had limited success. Predominantly PVC based latices generally have only a marginal advantage in flammability over non-flame retarded analogs. Addition of more chlorine in the form of vinylidene chloride has been quite limited due to high cost.
Curable resin compositions containing a basic catalyst and a water solution of polymerized halogen-containing vinyl monomer and other vinyl monomers are disclosed in Japanese Pat. No. 56120754-A2, issued to Mitsui Toatsu Chemicals on Sep. 22, 1981. The Mitsui patent reports that water based suspensions or emulsions of vinyl polymers have weak resistance to water, cracking and soiling (staining), and that the proposed compositions overcome such shortcomings. The patent mentions various halogenated vinyl monomers, including brominated monomers, but does not disclose the use of polybrominated monomers.
Moreover, the Mitsui patent is limited to treatment of water, cracking and soiling properties. No recognition is contained in the Mitsui patent of the preparation of flame retardant latices utilizing brominated vinyl monomers, and the patent fails to disclose percentages of use for such monomers to achieve flame retardancy. The patent proposes that the halogen-containing monomer comprise at most 15% by weight of the copolymer, which corresponds to a bromine content in the resin of at most about 6%. The Mitsui patent further indicates that it is preferred to have a lower percentage of halogen-containing monomer of at most 10% by weight, corresponding to a bromine content of at most 4%. These percentages are insufficient to provide desirable flame retardancy. In preferring the lower bromine content, the Mitsui patent teaches away from the present invention.
Bromine-containing plastics are described in European Patent Application No. 79200768.4, filed by Stamicarbon B.V. on Dec. 15, 1979 (published Sep. 7, 1980 as No. 13,052-A1). The Stamicarbon application is directed to the preparation of plastic materials, including polyolefins, polystyrene, and copolymers of styrene and butadiene, styrene and acrylonitrile and ABS. The plastics of the Stamicarbon application require high levels of bromine, and are described as containing 20-44 weight percent of bromine.
Moreover, the polymer compositions in the Stamicarbon patent are not suitable as coating materials because they have inappropriately high glass transition temperature (Tg) values. Their materials would have "excessive stiffness and inability to form film properly at room temperature." In fact, they teach away from the present invention.
Charles R. Martens wrote ("Waterborne Coatings, Emulsion and Water-Soluble Paints", Reinhold, N.Y., 1981, p. 169): "Choice of monomers in emulsion polymerization reactions is largely determined by the end-use requirements of the polymer. The first factor to be taken into account in the choice of a monomer or monomer mixture is the record order or glass transition temperature (Tg) of the polymer desired. This is the characteristic temperature at which the system undergoes a change from a hard, brittle material to a softer, more flexible one. Since polymers are generally unable to form films from latexes at temperatures below the Tg, an obvious requirement is that the polymer be above the Tg at the application and use temperature. A further restriction is placed on the Tg of the polymer by the fact that polymers become very soft at temperatures too far above the Tg, resulting in poor hardness, blocking, abrasion resistance, dirt collection, and so on. A balance must be obtained, therefore, between the good flexibility, adhesion, coalescence, and so forth, obtained at temperatures farther above the Tg and the good mechanical resistance properties found closer to the Tg. This is generally accomplished in coatings intended for ambient use by using polymers having Tg's in the 0.degree.-30.degree. C. range."
According to the above principle, Tg must be lower than the temperature at which film formation is attempted. If we choose the polymer with the lowest Tg of any described in the Stamicarbon patent, the composition is 60 parts by wt. bromostyrene (118.degree. C. Tg), 15 parts acrylonitrile (110.degree. C.), and 25 parts octyl acrylate (lowest Tg at -70.degree. C. of esters of acrylic acid). The Tg of this composition is 43.7.degree. C., according to the following equation: ##EQU1## Where Tg=multipolymer Tg in .degree. K., Wn=weight fraction of the monomer present and Tgn=the homopolymer Tg in .degree. K.
The Tg of the above composition, 43.7.degree. C., in the Stamicarbon's patent is recognized as too high to be of commercial value as a coating or a paint due to excessive stiffness and inability to form film properly at room temperature.
The Stamicarbon's compositions containing 60 to 95% bromostyrene, i.e., 26.2% Br to 41.5% Br, are outside those of the current invention (7-20% Br). Criteria for monomer selection include the glass transition temperature (Tg), physical properties, and chemical resistance. It is clear that the Stamicarbon's polymer composition cannot be such as recited by the present invention. Further, one skilled in the art would not utilize the latex of Stamicarbon as a coating or a paint to render articles nonflammable.
There has remained a need for polymer latex coatings which possess desired flame retardant and film properties. The coatings of the present invention satisfy these needs, and provide useful fire-retardant fabric backcoatings, paints, adhesives, sealants, caulking, non-woven binders and a variety of other applications.